Burr formation is an unavoidable consequence of every conventional material removal process. Burrs are fragments of built-up edge left on a workpiece edge during metal cutting. Some ways in which burrs can be formed include plastic deformation of the workpiece, tearing of metal chips from the workpiece, and stock separation in advance of completion of the cutting operation.
The increasing complexity and precision requirements of mechanical products, such as aircraft engines, has created a need for improved methods for controlling the surface texture and finish of manufactured parts. Aircraft engine parts depend upon proper finish of critical surfaces for their successful operation as well as for long trouble-free performance in service. Accordingly, burr removal, or deburring, is an extremely important procedure in the manufacture of jet engine components.
In the past, most deburring was performed manually due to the high degree of dexterity the procedure requires. Manual deburring has proved to be an expensive, labor intensive, monotonous, time consuming job which produces an inconsistent finished surface. More recently, deburring tools have been coupled with industrial robots in an attempt to improve product quality and increase production.
Deburring is a micromachining process which is extremely difficult for a robot to perform due to the high degree of flexibility and dexterity required. Several automated edge contouring or deburring systems have been designed to debur precision machined parts. The simplest systems comprise a grinding tool secured to a robot arm, while more sophisticated systems may include computer controlled robot systems, force sensors and feedback control schemes.
Initially, robotic systems were used for deburring casting and forging flash and for grinding weld beads--procedures characterized by low edge finish specifications. Precision parts, having more stringent edge finish specifications, presented problems in automating burr removal. Control scheme time lags and positional inaccuracy of robotic systems prevented formation of a high quality, burr-free, edge finish.
Closed loop robotic deburring systems provide feedback control schemes and contain sensors for monitoring either the position of the robot's arm or cutting forces during deburring. The sensor signals are continuously compared with the preprogramed position or force instructions for the robot arm, and are used to generate an error signal which is fed back to the controller. The controller adjusts the positioning of the robot arm to reduce the magnitude of the error signal. An unfortunate drawback of a closed loop feedback control scheme is the inability of the robot to respond in real time to the detailed features of a burr. The action of the deburring system in response to the error signal will always suffer a time delay due to the electrical and mechanical response times of the system. The position of the deburring tool will oscillate about the desired position as a result of this delay.
One simple solution to the problem of time delay associated with a closed loop feedback control scheme is the utilization of an open loop control system. In a simple open loop system, the desired tool path is programmed into the robot's controller and is independent of output measurements. A disadvantage of an open loop system is its inability to alter tool path in response to variations in part geometry or to part misalignment.
Other areas of concern with robotic deburring systems include positional accuracy, repeatability, machine rigidity, and tool wear compensation. Positional accuracy is the ability of the robot to position the deburring tool at a point which has been programmed into the machine controller. Repeatability is the ability of the robot to achieve the same tool position over and over again. Accuracy and repeatability are affected by such factors as machine resolution, friction between the moving parts of the robot arm, and time delays inherent in closed loop control schemes. Machine rigidity, or stiffness, refers to the robot's ability to resist bending or twisting in response to the different forces acting upon the apparatus. Tool wear compensation describes the ability of the system to adjust itself for gradual changes in cutting tool geometry due to wear.